What will be the temperature rise of the gas when the box hits the ground?

Question

An insulated box containing 0.5 kg of a gas having C_v = 0.98 kJ/kg. K falls from a balloon 4 km above the earth’s surface. What will be the temperature rise of the gas when the box hits the ground?

a. 0 K                                  b. 20 K

c. 40 K                                 d. 60 K